所获，不止Oracle之B-Tree目录
所获，不止Oracle之B-Tree索引
/*
索引
索引由三部分组成，根块(root),Branch(茎块)，Leaf(叶子块)。其中Leaf块主要存储了key column value(索引列具体值)，以及能具体
定位到数据块位置的rowid。
索引的创建过程：
1.要建索引先排序，将索引列的值顺序取出，及该行的rowid放入到内存中。
2.依次将内存中的顺序存放的值和rowid存放入索引块中。
3.当填满两个索引块后，oracle会产生一个块，用于管理同级的叶子块。这个块记录了叶子块的信息，并不记录索引列的键值，所以使用的空间比较少。
4.当管理叶子块的块被填满后，oracle又会产生一个上一级管理块，依次循环。同级两块需要管理。
索引结构的三大重要特点
1.索引的高度比较低
2.索引存储列值
3.索引本身是有序的
*/
----------------------------------------------索引高度较低验证-----------------------------------------
--索引的大小和高度是巨大差别的，可能大小差好多倍，但高度却一样。
--构造T1-T7表，记录从5到500W
CREATE TABLE T1 AS SELECT ROWNUM AS ID,ROWNUM+1 AS ID2 FROM DUAL CONNECT BY LEVEL<=5;
CREATE TABLE T2 AS SELECT ROWNUM AS ID,ROWNUM+1 AS ID2 FROM DUAL CONNECT BY LEVEL<=50;
CREATE TABLE T3 AS SELECT ROWNUM AS ID,ROWNUM+1 AS ID2 FROM DUAL CONNECT BY LEVEL<=500;
CREATE TABLE T4 AS SELECT ROWNUM AS ID,ROWNUM+1 AS ID2 FROM DUAL CONNECT BY LEVEL<=5000;
CREATE TABLE T5 AS SELECT ROWNUM AS ID,ROWNUM+1 AS ID2 FROM DUAL CONNECT BY LEVEL<=50000;
CREATE TABLE T6 AS SELECT ROWNUM AS ID,ROWNUM+1 AS ID2 FROM DUAL CONNECT BY LEVEL<=500000;
CREATE TABLE T7 AS SELECT ROWNUM AS ID,ROWNUM+1 AS ID2 FROM DUAL CONNECT BY LEVEL<=5000000;
--创建索引
CREATE INDEX IDX_ID_T1 ON T1(ID);
CREATE INDEX IDX_ID_T2 ON T2(ID);
CREATE INDEX IDX_ID_T3 ON T3(ID);
CREATE INDEX IDX_ID_T4 ON T4(ID);
CREATE INDEX IDX_ID_T5 ON T5(ID);
CREATE INDEX IDX_ID_T6 ON T6(ID);
CREATE INDEX IDX_ID_T7 ON T7(ID);
--查看索引大小
SELECT SEGMENT_NAME,BYTES/1024 FROM DBA_SEGMENTS WHERE SEGMENT_NAME IN
(
'IDX_ID_T1',
'IDX_ID_T2',
'IDX_ID_T3',
'IDX_ID_T4',
'IDX_ID_T5',
'IDX_ID_T6',
'IDX_ID_T7'
);
--查看索引高度
SELECT INDEX_NAME,
BLEVEL,--索引高度,BLEVEL=0表示1层,BLEVEL=1表示2层
LEAF_BLOCKS,--Number of leaf blocks in the index
NUM_ROWS,
DISTINCT_KEYS,
CLUSTERING_FACTOR
FROM USER_IND_STATISTICS
WHERE TABLE_NAME IN ('T1', 'T2', 'T3', 'T4', 'T5', 'T6', 'T7');
INDEX_NAME BLEVEL LEAF_BLOCKS NUM_ROWS DISTINCT_KEYS CLUSTERING_FACTOR
----------------------- ----------- ---------- ------------- -----------------
IDX_ID_T1 0 1 5 5 1
IDX_ID_T2 0 1 50 50 1
IDX_ID_T3 1 2 500 500 1
IDX_ID_T4 1 11 5000 5000 9
IDX_ID_T5 1 110 50000 50000 101
IDX_ID_T6 2 1113 500000 500000 1035
IDX_ID_T7 2 12023 5134040 4994100 22527
--相比T6表与T7表的查询效率，使用索引的查询效率一致，因为索引的高度低
--以下内容已多次执行消除 物理读和递归
admin@ORCL> select * from T6 where id = 10;
已用时间: 00: 00: 00.07
执行计划
----------------------
Plan hash value: 1902844584
-------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time
-------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 9 | 4 (0)| 00:00:
| 1 | TABLE ACCESS BY INDEX ROWID| T6 | 1 | 9 | 4 (0)| 00:00:
|* 2 | INDEX RANGE SCAN | IDX_ID_T6 | 1 | | 3 (0)| 00:00:
-------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("ID"=10)
统计信息
----------------------
0 recursive calls
0 db block gets
5 consistent gets
0 physical reads
0 redo size
462 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
admin@ORCL> select * from T7 where id = 10;
已用时间: 00: 00: 00.01
执行计划
----------------------
Plan hash value: 1124755243
-----------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 11 | 4 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T7 | 1 | 11 | 4 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IDX_ID_T7 | 1 | | 3 (0)| 00:00:01 |
-----------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("ID"=10)
统计信息
----------------------
0 recursive calls
0 db block gets
5 consistent gets
0 physical reads
0 redo size
462 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
--DUMP索引结构，一探究竟
admin@ORCL> column object_name for a20
admin@ORCL> SELECT OBJECT_ID, OBJECT_NAME FROM DBA_OBJECTS WHERE OBJECT_NAME IN ('IDX_ID_T1');
OBJECT_ID OBJECT_NAME
---------- --------------------
65143 IDX_ID_T1
--DUMP后存放的trace文件后缀
SELECT SPID
FROM V$PROCESS
WHERE ADDR = (SELECT PADDR
FROM V$SESSION
WHERE SID = (SELECT DISTINCT SID FROM V$MYSTAT));
SPID
------------
6704
--DUMP索引"结构"
alter system set events 'immediate trace name treedump level 65143';
--存放在以22408结尾的trace文件中
SELECT SPID
FROM V$PROCESS
WHERE ADDR = (SELECT PADDR
FROM V$SESSION
WHERE SID = (SELECT DISTINCT SID FROM V$MYSTAT));
--索引结构如下：
--只有一个叶子节点 0x代表16进制数，16798212是前面16进制转化为10进制的数
--以下内容记录了leaf对应的文件号和块号
----- begin tree dump
leaf: 0x1005204 16798212 (0: nrow: 5 rrow: 5)
----- end tree dump
--将16进制为10进制的数
admin@ORCL> select to_number('1005204','xxxxxxx') from dual;
TO_NUMBER('1005204','XXXXXXX')
------------------------------
16798212
--获取数据块的文件编号,及块编号
admin@ORCL> SELECT DBMS_UTILITY.DATA_BLOCK_ADDRESS_FILE(16798212) FROM DUAL;
DBMS_UTILITY.DATA_BLOCK_ADDRESS_FILE(16798212)
----------------------------------------------
4
admin@ORCL> SELECT DBMS_UTILITY.DATA_BLOCK_ADDRESS_BLOCK(16798212) FROM DUAL;
DBMS_UTILITY.DATA_BLOCK_ADDRESS_BLOCK(16798212)
-----------------------------------------------
20996
--从以下结果可以看出leaf块确实在分配的空间中
admin@ORCL> SELECT SEGMENT_NAME,BLOCK_ID,BLOCKS,RELATIVE_FNO FROM DBA_EXTENTS WHERE SEGMENT_NAME = 'IDX_ID_T1';
SEGMENT_NAME BLOCK_ID BLOCKS RELATIVE_FNO
-------------------- ---------- ---------- ------------
IDX_ID_T1 20993 8 4
--DUMP 数据块
ALTER SYSTEM DUMP DATAFILE 4 BLOCK 20996;
--DUMP内容如下
row#0[8024] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02 --十六进制
col 1; len 6; (6): 01 00 03 cc 00 00
row#1[8012] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 03
col 1; len 6; (6): 01 00 03 cc 00 01
row#2[8000] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 04
col 1; len 6; (6): 01 00 03 cc 00 02
row#3[7988] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 05
col 1; len 6; (6): 01 00 03 cc 00 03
row#4[7976] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 06
col 1; len 6; (6): 01 00 03 cc 00 04
----- end of leaf block dump -----
End dump data blocks tsn: 4 file#: 4 minblk 20996 maxblk 20996
--查看1转化为16进制的值为 c1,2
admin@ORCL> select dump(1,16) from dual;
DUMP(1,16)
-----------------
Typ=2 Len=2: c1,2
--4位二进制数表示16进制数 a在16进制中表示 10,b 代表 11,c为12 d为13
01 00 03 cc 00 00
0000 0001
0000 0000
0000 0011
1100 1100
0000 0000
0000 0000
--上面转化为二进制后，前十位为文件编号，后22个二进制数据为块编号，后面的数字为第几行
--从结果可以看出，索引内容中包含了索引列的值，及该条记录的rowid 后3部分内容。表段的OBJECT_ID是没有必要的
0000 0001 00: 4
00 0000 0000 0011 1100 1100 : 972
0000 0000 0000 0000: 0
SELECT ROWID, T1.* FROM T1;
admin@ORCL> SELECT DBMS_ROWID.ROWID_OBJECT(ROWID) OBJECT_ID,
2 DBMS_ROWID.ROWID_RELATIVE_FNO(ROWID) FILE_ID,
3 DBMS_ROWID.ROWID_BLOCK_NUMBER(ROWID) BLOCK_ID,
4 DBMS_ROWID.ROWID_ROW_NUMBER(ROWID) NUM
5 FROM T1;
OBJECT_ID FILE_ID BLOCK_ID NUM
---------- ---------- ---------- ----------
65136 4 972 0
65136 4 972 1
65136 4 972 2
65136 4 972 3
65136 4 972 4
--分区索引与全局索引的总结
--若建立的非前缀的索引，且无法有效的应用到分区条件，则最好建为全局索引。
--若考虑业务的查询只会集中在一个或少数几个分区，则最好建成分区索引
----------------------------------------------巧用索引存储列植-----------------------------------------
/*
总结：
sum/avg在是否为空都不会影响结果值，但Oracle必须列为非空才可以用到索引
max/min在列在非空或空，均可以用到索引
count(*)必须索引列不为空才可以，因为count(*)会统计空值.
count(column)时，不论column是否为空，都会用到此列的索引列。因为count(column)不统计空。
*/
--count
--什么时候索引扫描比全表扫描高效很多？
--当在大表上一个非空而且长度很短的列创建索引，这时索引的体积相对表来说特别小，那效率就高很多了。
DROP TABLE T PURGE;
CREATE TABLE T AS SELECT * FROM DBA_OBJECTS;
--没有应用到索引，因为object_id 可能为空
admin@ORCL> select count(*) from t;
执行计划
----------------------
Plan hash value: 2966233522
-------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-------------------------------
| 0 | SELECT STATEMENT | | 1 | 161 (2)| 00:00:02 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| T | 57075 | 161 (2)| 00:00:02 |
-------------------------------
--限制后为非空后，Oracle会使用到索引，因为索引不存储空值
admin@ORCL> SELECT COUNT(*) FROM T WHERE OBJECT_ID IS NOT NULL;
执行计划
----------------------
Plan hash value: 1131838604
---------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 13 | 30 (4)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
|* 2 | INDEX FAST FULL SCAN| IDX_OBJECT_ID | 57075 | 724K| 30 (4)| 00:00:01 |
---------------------------------------------------
--若是只统计object_id，Oracle会走索引，这点有点小奇怪，但经过测试，Oracle count单列的时候是不统计空值的，但统计count(*)时，
--哪怕整行都是空值，Oracle仍会记录此条记录。
admin@ORCL> SELECT COUNT(OBJECT_ID) FROM T;
执行计划
----------------------
Plan hash value: 1131838604
---------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 13 | 30 (4)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
| 2 | INDEX FAST FULL SCAN| IDX_OBJECT_ID | 57075 | 724K| 30 (4)| 00:00:01 |
---------------------------------------------------
--修改字段OBJECT_ID 为非空，Oracle则会自动应用OBJECT_ID上面的索引
ALTER TABLE T MODIFY OBJECT_ID NOT NULL;
admin@ORCL> SELECT COUNT(*) FROM T;
执行计划
----------------------
Plan hash value: 1131838604
-------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 30 (4)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | INDEX FAST FULL SCAN| IDX_OBJECT_ID | 57075 | 30 (4)| 00:00:01 |
-------------------------------------------
--SUM/AVG优化
DROP TABLE T PURGE;
CREATE TABLE T AS SELECT * FROM DBA_OBJECTS;
CREATE INDEX IDX_T_OBJECT_ID ON T(OBJECT_ID);
--SUM与AVG均没有用到索引，这个蛮奇怪，因为SUM与AVG的计算中均没有NULL记录的运算
admin@ORCL> SELECT SUM(OBJECT_ID) FROM T;
执行计划
----------------------
Plan hash value: 2966233522
---------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------
| 0 | SELECT STATEMENT | | 1 | 13 | 162 (2)| 00:00:02 |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
| 2 | TABLE ACCESS FULL| T | 57075 | 724K| 162 (2)| 00:00:02 |
---------------------------------------
admin@ORCL> SELECT AVG(OBJECT_ID) FROM T;
执行计划
----------------------
Plan hash value: 2966233522
---------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------
| 0 | SELECT STATEMENT | | 1 | 13 | 162 (2)| 00:00:02 |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
| 2 | TABLE ACCESS FULL| T | 57075 | 724K| 162 (2)| 00:00:02 |
---------------------------------------
--将OBJECT_ID设置为非空后，就可以用到索引。这点也蛮奇怪，可以sum是不包含空值的，但Oracle却不走索引。这点记住就好了
ALTER TABLE T MODIFY OBJECT_ID NOT NULL;
admin@ORCL> SELECT SUM(OBJECT_ID) FROM T;
执行计划
----------------------
Plan hash value: 1572773910
-----------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 13 | 30 (4)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
| 2 | INDEX FAST FULL SCAN| IDX_T_OBJECT_ID | 57075 | 724K| 30 (4)| 00:00:01 |
-----------------------------------------------------
admin@ORCL> SELECT AVG(OBJECT_ID) FROM T;
执行计划
----------------------
Plan hash value: 1572773910
-----------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 13 | 30 (4)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
| 2 | INDEX FAST FULL SCAN| IDX_T_OBJECT_ID | 57075 | 724K| 30 (4)| 00:00:01 |
-----------------------------------------------------
Note
-----
- dynamic sampling used for this statement
统计信息
----------------------
0 recursive calls
0 db block gets
119 consistent gets
0 physical reads
0 redo size
433 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
--执行计划可以看出，count,sum,avg连续三个聚合语句写在一块使用的资源与单个avg是一致的。这是因为一次扫描索引块，就可以同时解决三个问题。
admin@ORCL> SELECT SUM(OBJECT_ID),AVG(OBJECT_ID),COUNT(*) FROM T;
执行计划
----------------------
Plan hash value: 1572773910
-----------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 13 | 30 (4)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
| 2 | INDEX FAST FULL SCAN| IDX_T_OBJECT_ID | 57075 | 724K| 30 (4)| 00:00:01 |
-----------------------------------------------------
Note
-----
- dynamic sampling used for this statement
统计信息
----------------------
0 recursive calls
0 db block gets
119 consistent gets
0 physical reads
0 redo size
567 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
--测试MAX/MIN函数的优化,这时哪怕字段可以为NULL,仍可以用到索引
--INDEX FULL SCAN (MIN/MAX)：无论多大的索引，此操作都会非常快，因为索引是有序的，它只要去索引头块和索引尾块，即可查询到MIN/MAX值
ALTER TABLE T MODIFY OBJECT_ID NULL;
admin@ORCL> SELECT MAX(OBJECT_ID) FROM T;
执行计划
----------------------
Plan hash value: 1152835269
----------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 13 | 162 (2)| 00:00:02 |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
| 2 | INDEX FULL SCAN (MIN/MAX)| IDX_T_OBJECT_ID | 57075 | 724K| | |
----------------------------------------------------------
admin@ORCL> SELECT MIN(OBJECT_ID) FROM T;
执行计划
----------------------
Plan hash value: 1152835269
----------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 13 | 162 (2)| 00:00:02 |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
| 2 | INDEX FULL SCAN (MIN/MAX)| IDX_T_OBJECT_ID | 57075 | 724K| | |
----------------------------------------------------------
--此处没有用到索引,看到Oracle的优化器也不是太智能。
admin@ORCL> select max(object_id),min(object_id) from t;
执行计划
----------------------
Plan hash value: 2966233522
---------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------
| 0 | SELECT STATEMENT | | 1 | 5 | 162 (2)| 00:00:02 |
| 1 | SORT AGGREGATE | | 1 | 5 | | |
| 2 | TABLE ACCESS FULL| T | 50826 | 248K| 162 (2)| 00:00:02 |
---------------------------------------
--换种写法，就可以走索引了
SELECT MAX,MIN FROM (SELECT MAX(OBJECT_ID) MAX FROM T) A,(SELECT MIN(OBJECT_ID) MIN FROM T) B;
admin@ORCL> SELECT MAX,MIN FROM (SELECT MAX(OBJECT_ID) MAX FROM T) A,(SELECT MIN(OBJECT_ID) MIN FROM T) B
执行计划
----------------------
Plan hash value: 3758149690
------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 26 | 323 (2)| 00:00:04 |
| 1 | NESTED LOOPS | | 1 | 26 | 323 (2)| 00:00:04 |
| 2 | VIEW | | 1 | 13 | 162 (2)| 00:00:02 |
| 3 | SORT AGGREGATE | | 1 | 5 | | |
| 4 | INDEX FULL SCAN (MIN/MAX)| IDX_T_OBJECT_ID | 50826 | 248K| | |
| 5 | VIEW | | 1 | 13 | 162 (2)| 00:00:02 |
| 6 | SORT AGGREGATE | | 1 | 5 | | |
| 7 | INDEX FULL SCAN (MIN/MAX)| IDX_T_OBJECT_ID | 50826 | 248K| | |
------------------------------------------------------------
统计信息
----------------------
1 recursive calls
0 db block gets
4 consistent gets
--索引回表
--TABLE ACCESS BY INDEX ROWID，实际上是查询返回的值不在索引里面，索引需要根据rowid返回表，去查询得出。
--若查询的列可以在索引中查询到，则可以避免回表
--若需要查询多列，可以考虑建立复合索引，但要注意平衡，如果联合索引太多，必然导致索引过大
--虽然减少了回表动作，但增加了索引的大小，意味着查询就要遍历更多的索引块
--建议不要超过3个
admin@ORCL> select * from t where object_id = 20;
执行计划
----------------------
Plan hash value: 4285561625
-----------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 93 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T | 1 | 93 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IDX_T_OBJECT_ID | 1 | | 1 (0)| 00:00:01 |
-----------------------------------------------------------
admin@ORCL> select object_id from t where object_id = 20;
执行计划
----------------------
Plan hash value: 2498590897
------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 5 | 1 (0)| 00:00:01 |
|* 1 | INDEX RANGE SCAN| IDX_T_OBJECT_ID | 1 | 5 | 1 (0)| 00:00:01 |
------------------------------------------------
--Order by 之排序优化
--看来按字段排序，考虑用到索引，也会考虑到null值
SELECT * FROM T ORDER BY OBJECT_ID
admin@ORCL> SELECT * FROM T;
已选择50826行。
执行计划
----------------------
Plan hash value: 1601196873
--------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------
| 0 | SELECT STATEMENT | | 50826 | 4616K| 163 (3)| 00:00:02 |
| 1 | TABLE ACCESS FULL| T | 50826 | 4616K| 163 (3)| 00:00:02 |
--------------------------------------
统计信息
----------------------
0 recursive calls
0 db block gets
4058 consistent gets
0 physical reads
0 redo size
5460695 bytes sent via SQL*Net to client
37653 bytes received via SQL*Net from client
3390 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
50826 rows processed
--来看下没有排序，访问T表的资源
admin@ORCL> SELECT * FROM T ORDER BY OBJECT_ID;
已选择50826行。
执行计划
----------------------
Plan hash value: 961378228
-----------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
-----------------------------------------------
| 0 | SELECT STATEMENT | | 50826 | 4616K| | 1259 (2)| 00:00:16 |
| 1 | SORT ORDER BY | | 50826 | 4616K| 12M| 1259 (2)| 00:00:16 |
| 2 | TABLE ACCESS FULL| T | 50826 | 4616K| | 163 (3)| 00:00:02 |
-----------------------------------------------
统计信息
----------------------
0 recursive calls
0 db block gets
704 consistent gets
0 physical reads
0 redo size
2470336 bytes sent via SQL*Net to client
37653 bytes received via SQL*Net from client
3390 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
50826 rows processed
--查看段大小，发现只有6M，而排序段用了12M
admin@ORCL> SELECT OWNER, SEGMENT_NAME, BYTES / 1024 / 1024
2 FROM DBA_SEGMENTS
3 WHERE SEGMENT_NAME = 'T'
4 AND OWNER = 'ADMIN';
OWNER SEGMENT_NAME BYTES/1024/1024
------------------------------ -------------------- ---------------
ADMIN T 6
--来看下没有排序的资源消耗，cost为163,排序使用了1259,逻辑读为4058，而排序只使用了704.这点蛮奇怪，但
--真正决定性能的是cost的高低和真实完成的时间(cost右边的时间)，Oracle执行计划也是根据cost来决定执行路径
admin@ORCL> SELECT * FROM T;
已选择50826行。
执行计划
----------------------
Plan hash value: 1601196873
--------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------
| 0 | SELECT STATEMENT | | 50826 | 4616K| 163 (3)| 00:00:02 |
| 1 | TABLE ACCESS FULL| T | 50826 | 4616K| 163 (3)| 00:00:02 |
--------------------------------------
统计信息
----------------------
0 recursive calls
0 db block gets
4058 consistent gets
0 physical reads
0 redo size
5460695 bytes sent via SQL*Net to client
37653 bytes received via SQL*Net from client
3390 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
50826 rows processed
--看看索引的cost
ALTER TABLE T MODIFY OBJECT_ID NOT NULL;
--发现oracle第一次走索引扫描，也会有排序的动作
admin@ORCL> SELECT * FROM T ORDER BY OBJECT_ID;
已选择50826行。
执行计划
----------------------
Plan hash value: 2718353294
-----------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------
| 0 | SELECT STATEMENT | | 50826 | 4616K| 946 (1)| 00:00:12 |
| 1 | TABLE ACCESS BY INDEX ROWID| T | 50826 | 4616K| 946 (1)| 00:00:12 |
| 2 | INDEX FULL SCAN | IDX_T_OBJECT_ID | 50826 | | 115 (2)| 00:00:02 |
-----------------------------------------------------------
统计信息
----------------------
543 recursive calls
0 db block gets
7749 consistent gets
864 physical reads
0 redo size
5460695 bytes sent via SQL*Net to client
37653 bytes received via SQL*Net from client
3390 SQL*Net roundtrips to/from client
5 sorts (memory)
0 sorts (disk)
50826 rows processed
--第二次执行就没有排序动作咯，这点也是有点奇怪噻(5 sorts (memory))。但是走全表扫描排序时，会走SORT ORDER BY 步骤，此步骤会使用
--大约表大小的2倍去排序。此点比较消耗时间
--另外发现使用索引时，逻辑读会大很多，这个是因为走索引，只能通过单个块查询，而全表扫描可以读多个块。
admin@ORCL> SELECT * FROM T ORDER BY OBJECT_ID;
已选择50826行。
执行计划
----------------------
Plan hash value: 2718353294
-----------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------
| 0 | SELECT STATEMENT | | 50826 | 4616K| 946 (1)| 00:00:12 |
| 1 | TABLE ACCESS BY INDEX ROWID| T | 50826 | 4616K| 946 (1)| 00:00:12 |
| 2 | INDEX FULL SCAN | IDX_T_OBJECT_ID | 50826 | | 115 (2)| 00:00:02 |
-----------------------------------------------------------
统计信息
----------------------
0 recursive calls
0 db block gets
7666 consistent gets
0 physical reads
0 redo size
5460695 bytes sent via SQL*Net to client
37653 bytes received via SQL*Net from client
3390 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
50826 rows processed
--DISTINCT 排重优化
--大多数情况，使用索引消除重复，收效不是太明显。最好从业务上考虑，不保留重复。
UPDATE T SET OBJECT_ID =3 WHERE ROWNUM <=10000;
COMMIT;
--可以看出,虽然执行计划中，sorts为0.但HASH UNIQUE仍然使用了1M多的空间去重复。
admin@ORCL> SELECT DISTINCT OBJECT_ID FROM T;
已选择40827行。
执行计划
----------------------
Plan hash value: 1793979440
-----------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
-----------------------------------------------
| 0 | SELECT STATEMENT | | 50826 | 248K| | 325 (4)| 00:00:04 |
| 1 | HASH UNIQUE | | 50826 | 248K| 1208K| 325 (4)| 00:00:04 |
| 2 | TABLE ACCESS FULL| T | 50826 | 248K| | 162 (2)| 00:00:02 |
-----------------------------------------------
统计信息
----------------------
0 recursive calls
0 db block gets
704 consistent gets
0 physical reads
0 redo size
598769 bytes sent via SQL*Net to client
30316 bytes received via SQL*Net from client
2723 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
40827 rows processed
--修改T为非空
ALTER TABLE T MODIFY OBJECT_ID NULL;
--table access full变为index fast full scan,存在hash unique.
--cost提高不少。但执行计划没有去除排序
--index fast full scan 会一次性读取多个索引块，而index full scan扫描一次只能读取一个块.
--所以 index fast full scan无法排重,而index full scan是可以排重的。
--count(*)和sum无须用到排序，所以一般走 index fast null scan
admin@ORCL> SELECT DISTINCT OBJECT_ID FROM T;
已选择40827行。
执行计划
----------------------
Plan hash value: 2729247865
-------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
-------------------------------------------------------------
| 0 | SELECT STATEMENT | | 50826 | 248K| | 191 (5)| 00:00:03 |
| 1 | HASH UNIQUE | | 50826 | 248K| 1208K| 191 (5)| 00:00:03 |
| 2 | INDEX FAST FULL SCAN| IDX_T_OBJECT_ID | 50826 | 248K| | 27 (4)| 00:00:01 |
-------------------------------------------------------------
统计信息
----------------------
0 recursive calls
0 db block gets
186 consistent gets
0 physical reads
0 redo size
598769 bytes sent via SQL*Net to client
30316 bytes received via SQL*Net from client
2723 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
40827 rows processed
--强制走索引，效率不如上面
admin@ORCL> SELECT /*+ index(t) */DISTINCT OBJECT_ID FROM T;
已选择40827行。
执行计划
----------------------
Plan hash value: 503711260
--------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------
| 0 | SELECT STATEMENT | | 50826 | 248K| 278 (4)| 00:00:04 |
| 1 | SORT UNIQUE NOSORT| | 50826 | 248K| 278 (4)| 00:00:04 |
| 2 | INDEX FULL SCAN | IDX_T_OBJECT_ID | 50826 | 248K| 115 (2)| 00:00:02 |
--------------------------------------------------
统计信息
----------------------
0 recursive calls
0 db block gets
2864 consistent gets
0 physical reads
0 redo size
598769 bytes sent via SQL*Net to client
30316 bytes received via SQL*Net from client
2723 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
40827 rows processed
/*
几个索引的扫描方式：
INDEX FULLL SCAN：扫描一次只读取一个索引块
INDEX FAST FULL SCAN：一次性会读取多个索引块，读取多个数据块不容易保证有序。因此COUNT(*),SUM等不需要排序动作的操作会走INDEX FAST FULL SCAN
INDEX FULL SCAN(MIN/MAX)：索引扫描最大值和最小值
*/
--UNION优化
--看出来UNION会用到排序的步骤 SORT UNIQUE
admin@ORCL> SELECT OBJECT_ID FROM T
2 UNION
3 SELECT OBJECT_ID FROM T1;
已选择50890行。
执行计划
----------------------
Plan hash value: 631167089
---------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
---------------------------------------------------------------
| 0 | SELECT STATEMENT | | 101K| 894K| | 471 (62)| 00:00:06 |
| 1 | SORT UNIQUE | | 101K| 894K| 3216K| 471 (62)| 00:00:06 |
| 2 | UNION-ALL | | | | | | |
| 3 | INDEX FAST FULL SCAN| IDX_T_OBJECT_ID | 50826 | 248K| | 27 (4)| 00:00:01 |
| 4 | INDEX FAST FULL SCAN| IDX_T1_OBJECT_ID | 50898 | 646K| | 30 (4)| 00:00:01 |
---------------------------------------------------------------
Note
-----
- dynamic sampling used for this statement
统计信息
----------------------
0 recursive calls
0 db block gets
305 consistent gets
0 physical reads
0 redo size
736599 bytes sent via SQL*Net to client
37697 bytes received via SQL*Net from client
3394 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
50890 rows processed
--union all不会用到排序动作，对比下cost 471，而UNION ALL 的COST为57
admin@ORCL> SELECT OBJECT_ID FROM T1
2 UNION ALL
3 SELECT OBJECT_ID FROM T;
已选择101617行。
执行计划
----------------------
Plan hash value: 1727178076
------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------
| 0 | SELECT STATEMENT | | 101K| 894K| 57 (50)| 00:00:01 |
| 1 | UNION-ALL | | | | | |
| 2 | INDEX FAST FULL SCAN| IDX_T1_OBJECT_ID | 50898 | 646K| 30 (4)| 00:00:01 |
| 3 | INDEX FAST FULL SCAN| IDX_T_OBJECT_ID | 50826 | 248K| 27 (4)| 00:00:01 |
------------------------------------------------------
Note
-----
- dynamic sampling used for this statement
统计信息
----------------------
0 recursive calls
0 db block gets
7061 consistent gets
0 physical reads
0 redo size
1460206 bytes sent via SQL*Net to client
74899 bytes received via SQL*Net from client
6776 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
101617 rows processed
--尝试使用HINT去消除UNION 尝试消除排序段.发现并没有消除掉
--这是会因为两个结果集的筛选，各自的索引当然无法奏效。
admin@ORCL> SELECT /*+ INDEX (T) */OBJECT_ID FROM T
2 UNION
3 SELECT /*+ INDEX (T1) */OBJECT_ID FROM T1;
已选择50890行。
执行计划
----------------------
Plan hash value: 2084608915
----------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
----------------------------------------------------------
| 0 | SELECT STATEMENT | | 101K| 894K| | 654 (59)| 00:00:08 |
| 1 | SORT UNIQUE | | 101K| 894K| 3216K| 654 (59)| 00:00:08 |
| 2 | UNION-ALL | | | | | | |
| 3 | INDEX FULL SCAN| IDX_T_OBJECT_ID | 50826 | 248K| | 115 (2)| 00:00:02 |
| 4 | INDEX FULL SCAN| IDX_T1_OBJECT_ID | 50898 | 646K| | 126 (2)| 00:00:02 |
----------------------------------------------------------
Note
-----
- dynamic sampling used for this statement
统计信息
----------------------
0 recursive calls
0 db block gets
261 consistent gets
0 physical reads
0 redo size
736599 bytes sent via SQL*Net to client
37697 bytes received via SQL*Net from client
3394 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
50890 rows processed
--索引之主外键设计
CREATE TABLE T_P(ID NUMBER,NAME VARCHAR2(30));
--创建主键
ALTER TABLE T_P ADD CONSTRAINT T_P_ID_PK PRIMARY KEY(ID);
CREATE TABLE T_C(ID NUMBER,FID NUMBER,NAME VARCHAR2(30));
--创建外键约束
ALTER TABLE T_C ADD CONSTRAINT FK_T_C FOREIGN KEY(FID) REFERENCES T_P(ID);
--INSERT 数据
INSERT INTO T_P SELECT ROWNUM,TABLE_NAME FROM ALL_TABLES;
INSERT INTO T_C SELECT ROWNUM,MOD(ROWNUM,1000)+1,OBJECT_NAME FROM ALL_OBJECTS;
--看下两表join的执行计划
admin@ORCL> SELECT A.ID,A.NAME,B.NAME FROM T_P A,T_C B WHERE A.ID = B.FID AND A.ID = 880;
已选择50行。
执行计划
----------------------
Plan hash value: 727955870
------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------
| 0 | SELECT STATEMENT | | 39 | 2340 | 72 (3)| 00:00:01 |
| 1 | NESTED LOOPS | | 39 | 2340 | 72 (3)| 00:00:01 |
| 2 | TABLE ACCESS BY INDEX ROWID| T_P | 1 | 30 | 1 (0)| 00:00:01 |
|* 3 | INDEX UNIQUE SCAN | T_P_ID_PK | 1 | | 1 (0)| 00:00:01 |
|* 4 | TABLE ACCESS FULL | T_C | 39 | 1170 | 71 (3)| 00:00:01 |
------------------------------------------------------
--在T_C表上创建索引，再来看下执行计划
--
CREATE INDEX IDX_T_C_FID ON T_C(FID);
admin@ORCL> SELECT A.ID,A.NAME,B.NAME FROM T_P A,T_C B WHERE A.ID = B.FID AND A.ID = 880;
已选择50行。
执行计划
----------------------
Plan hash value: 4290308465
--------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------
| 0 | SELECT STATEMENT | | 50 | 2900 | 54 (0)| 00:00:01 |
| 1 | NESTED LOOPS | | 50 | 2900 | 54 (0)| 00:00:01 |
| 2 | TABLE ACCESS BY INDEX ROWID| T_P | 1 | 30 | 2 (0)| 00:00:01 |
|* 3 | INDEX UNIQUE SCAN | T_P_ID_PK | 1 | | 1 (0)| 00:00:01 |
| 4 | TABLE ACCESS BY INDEX ROWID| T_C | 50 | 1400 | 52 (0)| 00:00:01 |
|* 5 | INDEX RANGE SCAN | IDX_T_C_FID | 50 | | 1 (0)| 00:00:01 |
--------------------------------------------------------
--分析下外键约束的危害
--若没有在外键上创建索引，则在做DML操作外键所在的表时，会锁住整个主键表.
--删除索引
DROP INDEX IDX_T_C_FID;
--删除外键所在的表一条记录，会造成主键所在的表全表锁住。
admin@ORCL> DELETE T_C WHERE ID = 2;
已删除 1 行。
--执行任何DML都会锁住
admin@ORCL> DELETE T_P WHERE ID =2000;
--创建索引后，试试看.
--这样后，就不会锁住主键所在的表。
CREATE INDEX IDX_T_C_FID ON T_C(FID);
admin@ORCL> DELETE T_C WHERE ID = 2;
已删除 1 行。
admin@ORCL> DELETE T_P WHERE ID =2000;
已删除0行。
--尝试删除主键的表的记录
admin@ORCL> DELETE T_P WHERE ID =2;
DELETE T_P WHERE ID =2
*
第 1 行出现错误:
ORA-02292: 违反完整约束条件 (ADMIN.FK_T_C) - 已找到子记录
--指定ON DELETE CASCADE
ALTER TABLE T_C DROP CONSTRAINT FK_T_C;
ALTER TABLE T_C ADD CONSTRAINT FK_T_C FOREIGN KEY(FID) REFERENCES T_P(ID) ON DELETE CASCADE;
admin@ORCL> DELETE T_P WHERE ID =2;
已删除 1 行。
--再看组合索引
/*
1.适合的场合能避免回表
2.组合列返回越少越高效（过多的字段建立组合索引往往是不可取的，这样索引也必然过大，不宜超过三个）
3.组合索引，对于性能来将，谁放在前面都一样。
4.当时范围查询与等值查询结合时，等值查询列在前，范围查询列在后，这样的组合索引才高效
5.当只是范围查询时，肯定是范围查询的列在前时，查询效率高。
6.建立组合索引时，要考虑单列查询的情况，要把经常查询的列放在组合索引的第一列
*/
--测试下第三点
DROP TABLE T PURGE;
CREATE TABLE T AS SELECT * FROM DBA_OBJECTS;
CREATE INDEX IDX1_OBJECT_ID ON T(OBJECT_ID,OBJECT_TYPE);
CREATE INDEX IDX2_OBJECT_ID ON T(OBJECT_TYPE,OBJECT_ID);
admin@ORCL> SELECT * FROM T WHERE OBJECT_ID = 20 AND OBJECT_TYPE = 'TABLE';
执行计划
----------------------
Plan hash value: 1913591113
----------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 177 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T | 1 | 177 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IDX2_OBJECT_ID | 1 | | 1 (0)| 00:00:01 |
----------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("OBJECT_TYPE"='TABLE' AND "OBJECT_ID"=20)
统计信息
----------------------
0 recursive calls
0 db block gets
4 consistent gets
0 physical reads
0 redo size
1198 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
--利用hint，测试第二个索引
admin@ORCL> SELECT /*+INDEX(T,IDX1_OBJECT_ID)*/* FROM T WHERE OBJECT_ID = 20 AND OBJECT_TYPE = 'TABLE';
执行计划
----------------------
Plan hash value: 2486998213
----------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------
| 0 | SELECT STATEMENT | | 5 | 885 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T | 5 | 885 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IDX1_OBJECT_ID | 1 | | 1 (0)| 00:00:01 |
----------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("OBJECT_ID"=20 AND "OBJECT_TYPE"='TABLE')
Note
-----
- dynamic sampling used for this statement
统计信息
----------------------
0 recursive calls
0 db block gets
4 consistent gets
0 physical reads
0 redo size
1198 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
--看看组合索引对单列查询的影响
DROP INDEX IDX2_OBJECT_ID;
--会用到索引，当查询列在组合索引的前列时
admin@ORCL> SELECT * FROM T WHERE OBJECT_ID = 12;
执行计划
----------------------
Plan hash value: 2486998213
----------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 177 | 3 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T | 1 | 177 | 3 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IDX1_OBJECT_ID | 1 | | 2 (0)| 00:00:01 |
----------------------------------------------------------
DROP INDEX IDX1_OBJECT_ID;
--创建非前缀索引看看,发现并不会走索引
CREATE INDEX IDX2_OBJECT_ID ON T(OBJECT_TYPE,OBJECT_ID);
admin@ORCL> SELECT * FROM T WHERE OBJECT_ID = 12;
执行计划
----------------------
Plan hash value: 1601196873
--------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------
| 0 | SELECT STATEMENT | | 8 | 1416 | 162 (2)| 00:00:02 |
|* 1 | TABLE ACCESS FULL| T | 8 | 1416 | 162 (2)| 00:00:02 |
--------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("OBJECT_ID"=12)
统计信息
----------------------
0 recursive calls
0 db block gets
704 consistent gets
0 physical reads
0 redo size
1193 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
--强制走下索引，对比下效率。发现并不如全表扫描的效率，因为全表扫描会有多块读。
admin@ORCL> SELECT /*+index(t,IDX2_OBJECT_ID)*/* FROM T WHERE OBJECT_ID = 12;
执行计划
----------------------
Plan hash value: 961656401
----------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------
| 0 | SELECT STATEMENT | | 8 | 1416 | 187 (2)| 00:00:03 |
| 1 | TABLE ACCESS BY INDEX ROWID| T | 8 | 1416 | 187 (2)| 00:00:03 |
|* 2 | INDEX FULL SCAN | IDX2_OBJECT_ID | 190 | | 177 (2)| 00:00:03 |
----------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("OBJECT_ID"=12)
filter("OBJECT_ID"=12)
Note
-----
- dynamic sampling used for this statement
统计信息
--------------------------------------
0 recursive calls
0 db block gets
177 consistent gets
0 physical reads
0 redo size
1197 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
--有序插入与无序插入的执行时间
--当记录有序插入时，索引块的扩展和批量重组是可以批量做的。而无序插入是无法使用批量的。
--所以无序插入执行的速度比有序插入慢很多.
/*
索引对DML语句的影响
1.对INSERT影响最大，有百害而无一利，只要有索引，插入就慢，越多越慢。
2.对DELETE语句来说，有好有坏。海量数据中定位删除少量记录时，这个条件列时索引列时必要的。但过多列有索引还是会影响明显。
因为其他列的索引也要因此被更新。在经常需要删除大量记录的时候，危害加剧。
3.对UPDATE语句危害最小，快速定位少量并更新的场景和DELETE类似。
但具体修改某列时候，不会触发其他索引列的维护。
另外在创建索引的过程中，会产生锁，并把整个表锁住。任何该表的DML操作都将会被阻止。
这是因为建索引时，需要把索引列的列值全部取出来，加上锁是为了避免此时的列值被更新。
*/
--如何监控索引
alter index index_name monitoring usage;
--查看索引使用情况，进行跟踪
select * from v$object_usage;
--停止监控
alter index index_name nomonitoring usage;